1. Field of the Invention
This invention generally relates to electrochemical batteries and, more particularly, to a water-soluble binder for use with a transition. metal hexacyanometallate electrode.
2. Description of the Related Art
Transition metal cyanometallates (TMCMs) with large interstitial spaces have been investigated as the cathode material for rechargeable lithium-ion batteries [1, 2], sodium-ion batteries [3, 4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates ((Cu,Ni)-HCFs) exhibited a very good cycling life with 83% capacity retained after 40,000 cycles at a charge/discharge current of 17 C (1 C=150 milliamps per gram) [6-8]. However, the materials within the aqueous electrolyte demonstrated low capacities and energy densities because: (1) just one sodium-ion can be inserted/extracted into/from per Cu-HCF or Ni-HCF formula, and (2) these transition metal cyanoferrate (TM-HCF) electrodes must be operated below 1.23 V due to the water electrochemical window. The electrochemical window of a substance is the voltage range between which the substance is neither oxidized nor reduced. This range is important for the efficiency of an electrode, and once out of this range, water becomes electrolyzed, spoiling the electrical energy intended for another electrochemical reaction.
To correct these shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) were used as cathode materials in non-aqueous electrolyte [9, 10]. Assembled with a sodium-metal anode, Mn-HCF and Fe-HCF electrodes cycled between 2.0V and 4.2 V and delivered capacities of about 150 mAh/g.
Unlike conventional lithium-ion battery cathode materials, TMHCF can be easily prepared via precipitation in water, and does not require further high-temperature treatment. Parent applications Ser. Nos. 62/008,869 and 14/067,038, among others, describe exemplary precipitation synthesis, and are incorporated herein by reference. For example, Na2MnFe(CN)6 can be easily made by mixing two water solutions containing Na4Fe(CN)6 and MnCl2, which are subsequently filtered and dried at 100° C. Such an aqueous solution-based synthesis route provides an as-prepared TMHCF chemical having good stability and dispersion capability in water. Thus, TMHCM has a significantly lower synthesis cost as compared with the cathode materials used for lithium-ion batteries (LIBs). The low material cost of TMHCM makes it a very promising cathode material for stationary energy storage batteries, but the fabrication costs need to be cut even further to make it a truly viable battery option. Polyvinylidene fluoride (PVDF) is used as a standard binder for cathode electrode in LIBs because of its good adhesion and electrochemical stability. However, harmful organic solvents, like N-Methyl-2-pyrrolidone (NMP), are used to dissolve PVDF during the electrode coating process. A solvent recycling system is therefore required for cost and environment concerns. Thus, a high fabrication cost is associated with the conventional PVDF binder.
In contrast, a water-soluble binder is relatively inexpensive, process preferable, and environment friendly, all of which makes it a desirable binder for use in energy storage batteries. Although the substitution of PVDF with a water-soluble binder like carboxymethylcellulose (CMC) in LIBs has been investigated, challenges remain because the electrochemical performance of lithium transition metal oxides are compromised from dissolution or poor dispersion capability when aqueous binders are used. The electrode materials for lithium-ion batteries are prepared using high temperature calcinations, and problems typically occur when they are put into a water solution during electrode fabrication. For example, an ion-exchange reaction occurs between proton and lithium ions when LiMn2O4 is put into water. In other examples, the dissolution of active materials is observed when a LiNi1/3Co1/3Mn1/3O2 electrode is processed in an aqueous solution, and poor adhesion between LiFePO4 electrode and the current collector also hinders using water-soluble binders for battery fabrication.
It would be advantageous if a high quality electrode could be fabricated, with transition metal hexacyanometallate (TMHCM) as an active material and an aqueous binder, for use in sodium-ion batteries (SIBs) or other rechargeable metal-ion batteries.
[1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.
[2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron cyanide complex as positive electrode of lithium secondary battery, Journal of Power Sources, 79 (1999) 215-219.
[3] Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Prussian blue: a new framework for sodium batteries, Chemistry Communication, 48(2012)6544-6546.
[4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, J. B. Goodenough, A superior low-cost cathode for a Na-ion battery, Angew, Chem. Int. Ed., 52(2013)1964-1967.
[5] A. Eftekhari, Potassium secondary cell based on Prussian blue cathode, J. Power Sources, 126 (2004) 221-228.
[6] C. D. Wessels, R. A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2(2011) 550.
[7] C. D. Wessels, S. V. Peddada, R. A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries, Nano Letter, 11(2011) 5421-5425.
[8] C. D. Wessels, S. V. Peddada, M. T. McDowell, R. A. Huggins, Y. Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode, J. Electrochem. Soc., 159(2012) A98-A103.
[9] T. Matsuda, M. Takachi, Y. Moritomo, A sodium manganese ferrocyanide thin film for Na-ion batteries, Chemical Communications, DOI: 10.1039/C3CC38839E.
[10] S. -H. Yu, M. Shokouhimehr, T. Hyeon, Y. -E. Sung, Iron hexacyanoferrate nanoparticles as cathode materials for lithium and sodium rechargeable batteries, ECS Electrochemistry Letters, 2(2013)A39-A41.